Long lifetime mercury-metal halide discharge lamps

ABSTRACT

An improved mercury-metallic halide vapor arc discharge lamp in which sodium is present as a principal radiating specie and is added as the iodide to a mercury charge within a discharge envelope either solely or in the presence of other metallic iodides to produce a pleasing near-white emission, contains within the discharge envelope a sufficient quantity of elemental tin to prevent the existence of free iodine therein, thus achieving a greatly extended lifetime.

United States Patent [191 Field of Search 313/174, 229

Johnson Dec. 25, 1973 LQNG LIFETIME MERCURY-METAL Primary Examiner-Roy Lake I-IALIDE DISCHARGE LAMPS Assistant Examiner-Darwin R. Hostetter [75] Inventor: Peter D. Johnson, Schenectady, Attorney-John Ahem et N.Y. l 731 Assignee: General Electric Company, 57] ABSTRACT Schenectad N.Y.

y An improved mercury-metallic halide vapor are dis- Flled! 1972 charge lamp in which sodium is present as a principal [2]] Appl 311,943 radiating specie and is added as the iodide to a mercury charge within a discharge envelope either solely or in the presence of other metallic iodides to produce [52] 1.5. CI. 313/174, 3 l 3/229 a pleasing neapwhite emission, contains within the Ill. CI. discharge envelope 3 sufficient quantity of elemental [58] tin to prevent the existence of free iodine therein, thus achieving a greatly extended lifetime.

10 Claims, 1 Drawing Figure PAIEmmnEczsw The present invention relates to mercury-metallic halide vapor-arc discharge lamps of the type containing mercury and one or more vaporizable metallic halides which are dissociated in the heat of an are between a pair of primary arc electrodes extending into an evacuable envelope wherein the sodium atom is a principal radiating specie. More particularly, this invention relates to such vapor arc. discharge lamps wherein the presence of free iodinedue to chemical and related other mechanisms of clean-up of sodium from the discharge envelope is prevented and the life of the lamp is greatly extended.

Improved vapor arc discharge lamps for the production of high-intensity, high efficiency, near-white light have been made possible by the invention of the mercury-metallic halide vapor arc discharge lamp, as is set forth in U.S. Pat. No. 3,234,421, issued to G.I-l. Reiling, on Feb. 8, 1966 and assigned to the General Electric Company.

In accord with the teachings of the Reiling patent, the emission of a vapor-arc lamp is obtained from the line broadened spectra of one or more metals included within the lamp envelope as the halide thereof, preferably the iodide. In accord with the Reiling patent, sodium iodide is disclosed as the source of one principal radiating specie. The sodium atom as a principal radiating specie is particularly attractive sinee'the excitation potential of the sodium atom is one of the lowest of the materials which have been found to be suitable for additives in halide forms, principally as the iodide. Although the sodium atom emits an orange ed radiation which is substantially the line broadened characteristic of the sodium D lines, the addition of additional iodides as, for example, the iodides of thallium and indium, which emit in the green and blue portions of the visible spectrum, produces a, total emission wherein, the green and blue emissions blend with the emission of the so- 7 dium atom to produce a pleasing white, or near-white spectral rendition.

Although lamps of the type described utilizing sodium as a principal radiating specie are greatly advantageous, the chemical, activity of sodium and. other related effects cause the amount, of sodium within the discharge envelope progressively to. decrease with increasing time of lamp operation; Decreasing sodium content causes an increase in the voltage drop between the arc electrodes and a gradual change, of the spectral. emissivity in. the direction of the blue together with a decrease in efficacy, and constriction and instabilities of the vapor arc.

Accordingly, a. principal object of the present; invention is to, provide metallic vaporarc lamps in which sodium is a principal radiating specie having greatly in.- creased lifetimes.

Yet another object of the present invention is to provide lamps of the, type described, wherein long useful lifetime is achieved without .loss of efficacy or color rendition.

Still. another object of the present invention is to provide lamps of the typedescribed wherein-sodium is. a principal emitting specie in which the presence of free iodine; is prevented or minimized.

In accord withone embodiment of thepresentinvention, I provide a mercury-metallichalid'e vapor are discharge lamp having an evacuable envelope containing a pair of arc electrodes and a vaporizable charge ineluding a quantity of mercury, a quantity of sodium io dide, with or without other iodides, whose spectral emissivity blends with the emissivity of the sodium atom to provide white or near-white spectral rendition and a quantity of elemental thin sufficient to getter any free iodine which would be present within the lamp due to the clean-up of sodium from the sodium iodide charge, thus greatly extending the useful lifetime of the lamp without loss of spectral fidelity or substantial increase in voltage drop between the arc electrodes thereof.

The novel features characteristic of the present invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following detailed description taken in connection with the appended drawing in which the sole FIGURE is a vertical view, with parts broken away, of a lamp constructed in accord with the present invention.

In the drawing,-a mercury-metallic halide vapor-arc discharge lamp constructed in accord with the present invention includes an exterior evacuable transmissive envelope I mounted upon a screw-type contact-making base 2 and including therein an inner arc containing envelope 3. Inner envelope 3 is lighttransmissive, generally cylindrical in shape, hermetically sealed and is terminated at either end with pinched-off sections 4 and 5 which hermetically seal the interior thereof and through which thermionic arc electrodes 6 and 7 are suspended by means of in leads 8 and 9, respectively.

Envelope l may be any suitable high temperature, hard, light-transmissive glass as, for example, Pyrex or \(ycor. Envelope 3 may be any similar lighttransmissive, but higher temperature resistant material such as fused quartz, Lucalox, (U.S. Pat. No. 3,026,210) or high-density yttria, (Yttralox U.S. Pat. No. 3,545,987), or other similar high denisty, light-transmissive, or light-transparent ceramics. Since,

' in operation of lamps in accord with the presentinvention, the coolest portion of theinner wall of discharge envelope 3 is approximately at least 5009C, and preferably at least 600C, the hotter portions thereof immediately adjacentthe arc, generally are of the order of 1,000C, at least, and may be higher. Accordingly, envelope 3 should be of such materials as set forth hereinbefore as to withstand such temperatures without losing mechanical strength or suffering loss of transmissivity.

Arc electrodes 6 and 7 are spaced a predetermined distance apart depending upon the power rating of the lamp so as to sustain a high current electric are therebetween for the vaporization of the vaporizable constituents contained therein and the production of a highintensity radiation of appropriate wavelengths. A startingelectrode 10 is located within one end of inner envelope. 3i and is sealed through pinched region 5 of inner envelope thereof. Starting electrode 10 is connectedthrough a voltage dropping resistance 11 to a lead and support member 12 which is at the same electrical potential with another lead and support member l3',,both of which are connected to one contact member of connecting base 2. It will be appreciated, however, that other means than starting electrode 10 may be utilized to start the lamp.

light- Inner envelope 3 is suspended within outer envelope 1 by means of a simple set of saddle clamp members 14 and 15 which are dependent from lead and support member 13 and which are securely mechanically fastened about the flattened portions of pinched regions 4 and 5 of inner envelope 3. Lower saddle clamp 14 is connected between support members 12 and 13 and upper saddle clamp 15 is connected between support member 13 and a suspended support member 16 which is connected to a collar 17 which fits over a reentrant nipple 18 within the upper portion of exterior bulb member 1, which collar 16 serves to anchor the upper end of lead and support member 13. One arc electrode 6 is connected to support member 13, while the other are electrode 7 is connected to a separate lead 20, which is connected to one contact member of screw base 2 other than the one thereof to which support members 13 and 14 are connected. While the foregoing description of the support structure within outer envelope l is typical, it is by no means unique. Other means of support may be utilized, depending upon the power level at which the device is adapted to operate and the particular use to which the device is to be put. Additionally, lamps which are designed for horizontal mounting may differ from those which are designed for vertical operation. Alternatively, other constructions may be suitable for either vertical or horizontal mount- Discharge envelope 3 contains a starting gas as, for example, one of the noble gases such as argon, generally at a pressure of approximately 5 to 50 torr and preferably of approximately torr. This gas is utilized in starting the lamp by serving as the conducting specie of an initial are which vaporizes the main arc specie. Discharge envelope 3 also contains a charge 19 which containsthe vaporizable materials from which the light emitting specie are derived. Charge 19 is generally in the form of a globule containing mercury and one or more iodides, including sodium iodide, which are the iodides of the radiating metallic specie specifically chosen to produce the spectral emissivity desired from the lamp. For general purposes, it is desirable that white light be obtained and one such charge ideally suited to produce white light constitutes the iodides of sodium, thallium, and indium. Charge 19 also contains, in accord with the invention, a quantity of elemental tin. The quantity of mercury, the various iodides, and tin within charge 19 vary with the power rating of the lamp. As is to be expected, at the higher power rating of the lamp, the greater the volume of the discharge envelope 3, and the greater the spacing between arc electrodes 6 and 7. Typically, a 400 watt lamp may have a discharge envelope of approximately 20 cc, while for a 1,000 watt lamp the volume thereof. is approximately 35 cc, generally. irrespective of the power level of the lamp, however, the vapor pressure of the respective materials within the discharge envelope during operation of the lamp is usually within the same range. Thus, for example, mercury is present within the range of l to 20 atmospheres partial pressure. A sufficient quantity of mercury is therefore provided in the charge which, at lamp operating temperatures, (wherein the minimum interior bulb wall temperature is maintained at a value of at least 500C, and preferably at least 600C) is sufficient to provide a quantity of mercury vapor to establish such a pressure, leaving a residual amount of mercury in the charge with the unvaporized halides and tin. Sufficient sodium halide and any other halides utilized is added in order to produce a saturated vapor pressure of these materials at lamp operating temperatures. Such vapor pressure is generally of the order of 0.05 to 25 torr for sodium iodide, 0.1 lOO torr for indium iodide, and 0.1 to torr for thallium iodide.

One important mechanism which has often limited the useful lifetime of mercury-metallic vapor are discharge lamps, wherein sodium iodide is a principal source of sodium and sodium is a principal radiating specie, has been the extreme chemical and other activity of sodium which causes sodium to be depleted, or cleaned up, from the volume of the discharge envelope. While it is a seemingly possible solution to add an excess of sodium iodide to charge 19, in order to maintain a sufficient partial pressure of sodium iodide within the discharge envelope to continue the required amount of sodium necesary to give the desired spectral output, this is not a satisfactory solution to the problem. The reasons that merely adding additional sodium iodide to the charge does not solve the problem is that, with the clean-up of sodium from the discharge envelope, the iodine which was, prior to dissociation, a part of the sodium iodide in the vapor state within the envelope remains as free iodine. The free iodine within the discharge envelope causes constriction of the metallic vapor arc and increases the voltage drop between the arc electrodes while simultaneously causing local overheating of the discharge envelope wall.

Due to the foregoing deleterious effects of free iodine within the discharge envelope of lamps of this type, and the tendency to a clean-up of sodium, leaving a residue of free iodine, the general practice has, in the past, been to add only that amount of sodium iodide which is essential to yield a partial pressure of' sodium iodide in the discharge tube sufficient to yield enough free sodium upon dissociation to provide the requisite orangered emission for a reasonable operating lifetime. Such as operating lifetime has, in the past, been approximately 4,000 to 8,000 hours, depending upon many factors which influence clean-up of sodium 1 655406 watt mercury-metallic vapor-arc discharge lamp, a quantity of sodium of approximately '6 to 8 milligrams is sufficient for this purpose.

l have discovered that elemental tin provides'an excellent getter for free iodine produced by the clean-up of sodium from lamps of this type, wherein sodium is a principal radiating specie, and is added in the form of the iodide thereof. While many materials are known as iodine getters, no other material is known which is thermodynamically compatible with the operating parameters of mercury-metallic iodide vapor-arc lamps.

Thus, in accord with the present invention, free tin added to charge 19 is vaporized along with the iodides and mercury of charge 19 and, although the tin radiation is not significant as comapred with the sodium radiation, the elemental tin serves to maintain a thermodynamic equilibrium with the iodine freed by sodium clean-up so that, in the discharge envelope within and surrounding the metallic vapor arc, tin iodide, rather than free iodine, is present and constriction of the are, together with all of .the attendant disadvantages thereof, is avoided.

I am aware that tin iodide has previously been added to mercury-metallic arc lamps in .order to obtain the spectral emissivity of the tin atom, but since the addition of tin iodide to a charge within a discharge envelope brings' with it a stochiometric quantity of iodine, the presence of the tin added as the iodide thereof can in no way compensate for the depletion of any sodium which may be present, if at all, during lamp operation and the tin cannot therefore act as a getter for free iodine released thereby.

Lamps constructed in accord with the present invention show a remarkable improvement over similar lamps which do not contain elemental tin in the charge from which the light emitting specie is eventually evolved. Thus, for example, a standard prior art 400 watt mercury-metallic iodide vapor arc lamp, using sodium iodide, thallium iodide, and indium iodide, typically has a maximum useful normal lifetime of approximately 8,000 hours and may under normal performance specifications have a lifetime of as short as 4,000 hours. Lamps of the same rating constructed in accord with the present invention, on the other hand, routinely have lifetimes of 20,000 hours or greater. Mercurymetallic iodide lamps rated at 400 watts in which sodium is-a principal emitting specie normally have an initial voltage drop (for an interelectrode spacing of 40 mm) of approximately 135 volts. Prior art lamps, as is set forth hereinbefore, usually utilize a charge which typically contains 80 mg of mercury, 8 to 10 mg of sodium iodide, 2 mg of thallium iodide, 1 mg of indium iodide. and at: of a eas .Sush, lamswtth sndnftheiruseful life (at the expiration of from 4,000 to 8,000 hours operation) have a voltage drop between the arc electrodes of approximately 190 volts, the arc is severely constricted and often spiralling due to the presence of iodine vapors in the discharge envelope and tend to have a characteristic greenish cast. In-

' creased arc voltage causes increased power consumption and overheating, with numerous attendant deleterious effects.

.Lamps in accord with the present invention, on the other hand, although having the same. initial voltage drop between 40 mm spaced arc electrodes for a 400 watt lamp, never reach the unacceptably high value of 190 volts are drop, but rather exhibit a maximum voltage between the arc electrodes of approximately 1 50 volts maximum after approximately 20,000 hours of operation. At this time, the characteristic emission therefrom is still substantially white or near-white and only a minor amount of constriction may be found or observed 'in the are discharge. By this time, the lamp has usually reached the end of its useful life by virtue of wearing out of the cathodes or the rendering of the dischargetube opaque by sputtering of cathode material therefrom, or other normal wear-out phenomena.

As is mentionedhereinbefore, in prior art metallicmetal vapor arc lamps, wherein sodium is a principal emitting specie, the amount of sodium added is very strictly restricted in order to prevent earlyfailure by the evolution of a quantity of free iodine. This limitation on the amount of sodium iodide which maybe utilized is removed by theaddition of elementaltin to the charge in accord with the present invention. Thus, so long as there is elemental tin in the charge which may I be vaporized at the operating temperature of thelamp to combine with any iodine freed by clean-up of sodium, the saturation pressure of sodium which is approximately 0.05 to 25 torr, depending upon the operating temperature of the lamp, may be maintained within the discharge envelope and a sufficient quantity of sodium iodide may be added to the charge in order to continue the saturation pressure until the 20,000 hour lifetime of the lamp, due to the expiration of other components thereof, has arrived.

Generally, the amount of tin which'must he added to the charge is that which is sufficient over the operating lifetime of the lamp to prevent the existence within the discharge envelope of any free iodine. Thus, there must be enough free tin to form an amount of tin iodide, utilizing all of the iodine freed by the amount of sodium which is cleaned up in operation over such lifetime. Theoretically, in order to achieve the full effects of the present invention, a maximum of one-half mole of free tin should be added for each mole of sodium iodide present initially in charge 19. As a practical operating matter, sincea substantial excess of sodium iodide may now be used, there being a means of preventing the adverse effects of the creation of. free iodine and it being advisable to add an excess of sodium iodide to increase initial color rendition and to maintain color fidelity during the life of the lamp, a substantial amount of sodium added is not utilized nor cleaned up. Thus, as little as 0.1 mole of tin for each mole of sodium iodide may be utilized in the instances in which relatively large quantities of sodium iodide are added.

The amount of sodium iodide added to lamps in accord with the present invention generally range from approximately 2 to 6 milligrams thereof per cubic centimet er of discharge tube volume. Thus, for example, in a 400 watt lamp with a discharge tube volume of 20 cc, sodium iodide is present in the charge in a range of approximately 40 to 120 milligrams. The amount of mercury is generally chosen so as to be sufficient upon near total vaporization thereof to form a partial pressure of mercury in the discharge envelope of approximately 1 to 20 atmospheres. Since the amount of other halides added in lamps of this type as, for example, thallium iodide and indium iodide, are not involved in the free iodine and the sodium clean up mechanism, the amount of such other halides, specifically the iodides, may be added as is conventional in mercurymetallic iodide lamps in order to achieve the desired emission, generally white or near-white. Thus, for example, the amount of thallium iodide and indium iodide in such lamps may be such as to establish'a partial pressure thereof under lamp operating conditions of approximately 0.1 to torr each within the envelope.

Within the framework established by the foregoing general criteria, I find that in a 400 watt mercurymetallic arc vapor lamp, having 40 mm inner electrode spacings and a discharge envelope of 20 cubic centimeter volume, that a quantity of mercury of approximately 80 milligrams is sufficient to establish a partial pressure of 4 atmospheres under operating conditions. With this mercury charge, a quantity of from 40 to milligramsof sodium iodide may be added and a useful quantity of tin in the range of from 10 to 50 milligrams may typically be used. It should be appreciated, however, that the amount of tin added does not relate arithmetically to the amount of sodium iodide present by a simple application of the range of useful molar ratios. This is because the amount of tin added is chosen to be adequate to compensate for the amount of sodium cleaned up, not the total amount of sodium present. Thus, as set forth hereinbefore, with small quantities of sodium iodide present a larger molar fraction of tin is added than when a large quantity of sodium is added for spectral fidelity purposes. Thus, although many proportions of tin in relation to sodium iodide present may be chosen, the ultimate criterion is to provide sufficient tin to preclude the presence of free iodine within the discharge envelope during useful lamp life. Such a charge of the typical 400 watt mercury-metallic halide lamp may be completed by adding from 0.5 to 5.0 milligrams of thallium iodide and 0.5 to 5.0 milligrams of indium iodide. One such lamp utilized the following specific charge 80 mgs sodium iodide 80 mgs mercury 20 mgs tin 2 mgs thallium iodide 1 mg indium iodide 20 torr argon This lamp had an initial interelectrode voltage drop of 135 volts and, after 20,000 hours of operation, still had a relatively low interelectrode drop of 150 volts and emitted substantially white light with only barely observable constriction of the are.

In operation of lamps in accord with the present invention, a line voltage of 220 or 440 volts, for example, indicated by voltage source 21, isapplied to a conventional mercury-metallic halide ballast transformer, such as a General Electric Ballast Cat. No. 9SA20H1 l, 22 and applied by means of leads 23 and 24 to the base of the lamp and thence to the respective arc electrodes 6 and 7 and to starter electrode 10. An initial arc is striken between starter electrode 10 and main electrode 7, and is sufficient to cause the ionization of the low pressure inert gas within the envelope, thus facilitating a breakdown between main electrodes 6 and 7. Such an initial breakdown in the inert gas heats the mercury of the charge so that a sufficient amount of mercury is vaporized to cause a mercury discharge to be established between arc electrodes 6 and 7. As soon as high current conduction is initiated between are electrodes 6 and 7, the resistance 16 in the circuit to starter electrode is sufficient to extinguish the auxiliary are between starter electrode and main electrode 7. The mercury are established between the main electrodes 6 and 7 generates a sufficient quantity of heat to vaporize the respective halides, as, for example, the iodides of thallium, sodium, and indium, and elemental tin to cause the sodium, thallium, and indium to be dissociated and cause the establish-ment of a sodium, thallium, and indium arc discharge, for example.

In summary, by the foregoing, there has been disclosed an improvement in mercury metallic vapor arc lamps of the type utilizing sodium as a principal emitting specie wherein the sodium is added as the iodide thereof. The normal tendency of such lamps to have a constricted lifetime by virtue of the formation of free iodine vapor within the discharge envelope which is caused by clean-up of sodium, is avoided by adding to the charge containing mercury and sodium, with or without other iodides, a quantity of metallic tin varying from 0.1 to 0.5 moles of tin for each mole of sodium iodide added to the charge. Any iodine which tends to be released in the discharge envelope by virtue of clean-up of sodium is immediately combined with the tin vapor to form tin iodide which precludes the constriction of the arc discharge and attendant instabilities thereof, as well as preventing substantial increase of the interelectrode voltage drop and adverse heating of the discharge envelope, while at the same time maintaining color fidelity throughout the normal lifetime of the lamp as determined by the lifetime of other lamp constituents as, for example, the cathode.

Lamps constructed in accord with the present invention exhibit lifetimes of at least 20,000 hours, as compared with the 4,000 to 8,000 hours normally achievable with mercury-metallic vapor arc discharge lamps of the prior art.

While the invention has been set forth herein with respect to certain embodiments and specific examples thereof, many modifications and changes will readily occur to those skilled in the art. Accordingly, I intend by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A high pressure metal vapor electric discharge lamp comprising:

a. an hermetically sealed light transmissive discharge envelope;

b. a pair of primary arc-electrodes extending through opposite endwall portions of said discharge envelope and adapted to sustain a high current metallic vapor discharge therebetween;

c. a filling within said discharge envelope including,

c,. a low pressure of an inert gas, c a charge of at least mercury, sodium iodide, and

elemental tin,

d. means connected to said arc-electrodes for applying thereto a voltage having current capacity sufficient to establish an electric discharge within said lamp envelope which maintains the coolest portion of the inner wall thereof at a temperature of at least 500C to provide sufficient vaporization of said fill to provide high efficacy substantially white visible radiation.

2. The lamp of claim 1 wherein said mercury is present in a sufficient quantity as to provide within said envelope under operating conditions a vapor pressure thereof of from 1 to 20 atmospheres.

3. The lamp of claim 2 wherein said inert starting gas is present within said envelope at a partial pressure of approximately 5 to 50 torr.

4. The lamp of claim 2 wherein said sodium iodide is present in a sufficient quantity so as to provide under lamp operating conditions a partial pressure therein within said discharge envelope of approximately 0.05 to 25 torr and retain a substantial quantity thereof in the unvaporized state.

5. The lamp of claim 4 wherein said sodium iodide is present in a quantity of approximately 2 to 6 mgs per cubic centimeter of discharge envelope volume.

6. The lamp of claim 5 wherein said elemental tin is present in a quantity sufficient to combine with all free iodine evolved within said lamp during the lifetime thereof by the cleanup of sodium therefrom.

7. The lamp of claim 5 wherein said elemental tin is present in a quantity ranging from approximately 0.1 to 0.5 mole thereof for each of the mole of sodium iodide present therein.

8. The lamp of claim 4 wherein said sodium iodide is present in a quantity of approximately 2 to 6 mgs per cubic centimeter of the volume of said discharge envelope and said elemental tin is present in a quantity of approximately 0.1 to 0.5 mole for each mole of sodium iodide present.

9. The lamp of claim 4 wherein said charge contains at least one additional vaporizable dissociable halide of 10. The lamp of claim 4 wherein said discharge envelope also contains. quantities of thallium iodide and indium iodide sufficient to cause the spectral emission a light emitting metal which combines with the emis- 5 from Said p to be essentially White lightsion of sodium to provide a desired spectral rendition. 

2. The lamp of claim 1 wherein said mercury is present in a sufficient quantity as to provide within said envelope under operating conditions a vapor pressure thereof of from 1 to 20 atmospheres.
 3. The lamp of claim 2 wherein said inert starting gas is present within said envelope at a partial pressure of approximately 5 to 50 torr.
 4. The lamp of claim 2 wherein said sodium iodide is present in a sufficient quantity so as to provide under lamp operating conditions a partial pressure therein within said discharge envelope of approximately 0.05 to 25 torr and retain a substantial quantity thereof in the unvaporized statE.
 5. The lamp of claim 4 wherein said sodium iodide is present in a quantity of approximately 2 to 6 mgs per cubic centimeter of discharge envelope volume.
 6. The lamp of claim 5 wherein said elemental tin is present in a quantity sufficient to combine with all free iodine evolved within said lamp during the lifetime thereof by the cleanup of sodium therefrom.
 7. The lamp of claim 5 wherein said elemental tin is present in a quantity ranging from approximately 0.1 to 0.5 mole thereof for each of the mole of sodium iodide present therein.
 8. The lamp of claim 4 wherein said sodium iodide is present in a quantity of approximately 2 to 6 mgs per cubic centimeter of the volume of said discharge envelope and said elemental tin is present in a quantity of approximately 0.1 to 0.5 mole for each mole of sodium iodide present.
 9. The lamp of claim 4 wherein said charge contains at least one additional vaporizable dissociable halide of a light emitting metal which combines with the emission of sodium to provide a desired spectral rendition.
 10. The lamp of claim 4 wherein said discharge envelope also contains quantities of thallium iodide and indium iodide sufficient to cause the spectral emission from said lamp to be essentially white light. 